The present invention relates to a method of manufacturing an electrolytic capacitor which is arranged to prevent an electrolytic solution from drying up for increased reliability.
There have been known a variety of causes of deterioration of electrolytic capacitors. Since a factor of major concern involved in the deterioration of electrolytic capacitors is a loss of electrolytic solution, however, efforts should be made to improve electrolytic capacitors to minimize a loss of electrolytic solution for increased reliability.
One conventional method of manufacturing an electrolytic capacitor will be described below with reference to FIGS. 11A through lid and 12A through 12C of the accompanying drawings. FIGS. 11A through lid and 12A through 12C show in perspective various parts of an electrolytic capacitor in a succession of steps of a method of manufacturing the electrolytic capacitor.
As shown in FIG. 11A, a lead 1A in the form of an iron or copper wire and an attachment lB of aluminum with its end pressed to flat shape are welded to each other, thereby forming a lead terminal 1.
Then, as shown in FIG. 11B, the surface of an anode foil 2 of aluminum, for example, is etched into a roughened surface by a wet etching process with an etchant which comprises a solution of hydrochloric acid or a mixed solution of sulfuric acid, nitric acid, and acetic acid. The anode foil 2 is immersed in an electrolytic solution, and anodized to produce an oxide film thereon.
One lead terminal 1 is fixed to the anode foil 2 as by staking or ultrasonic welding, and another lead terminal 1 is fixed to a cathode foil 3 (see FIG. 11C) of aluminum, for example, as by staking or ultrasonic welding.
As shown in FIG. 11C, the anode foil 2, a separator 4 comprising a sheet of insulating paper, the cathode foil 3, and a separator 5 comprising a sheet of insulating paper are superposed with each other.
Then, as shown in FIG. 11D, they are coiled into a cylindrical shape, and fastened together by a tape 6, thereby producing a capacitor assembly 7.
Thereafter, the capacitor assembly 7 is set in a vacuum impregnator, which is then evacuated to remove air from the capacitor assembly 7. An electrolytic solution is then poured into the capacitor assembly 7. Since the surface of the anode foil 2 has been roughened, the introduced electrolytic solution finds its way into minute pores formed in the roughened surface of the anode foil 2.
Instead of placing the capacitor assembly 7 in the vacuum impregnator, the capacitor assembly 7 may simply be immersed in an electrolytic solution under the atmospheric pressure.
As shown in FIG. 12A, a sealing cap 8 of rubber is press-fitted over the leads 1A of the lead terminals 1, and then an aluminum case 9 is placed over the capacitor assembly 7. Then, as shown in FIG. 12B, engaging ends of the sealing cap 8 and the aluminum case 9 are curled into a seam 10, thus hermetically sealing the capacitor assembly 7.
If necessary, the aluminum case 9 is then covered with a heat-shrinkable tube 11 that has been printed with various items of information which generally include the name of the manufacturer, a capacitor type number, ratings, a lot number, and polarities, as shown in FIG. 12C.
The electrolytic capacitor thus produced is aged for about one hour with an applied voltage which is about 10% higher than the rated voltage and at a temperature which is an upper-limit temperature of the temperature range for its usage. The electrolytic capacitor is so aged as to make up defects produced primarily in the oxide film on the anode foil 2 during the fabrication process, based on the self-recovery capability of the electrolytic capacitor.
The aging conditions differ depending on the manufacturing conditions of the electrolytic capacitor, and vary from manufacturer to manufacturer.
Electrolytic capacitors manufactured according to the above method are inspected based on the product specifications, and any electrolytic capacitors which are found defective by the inspection process are rejected.
It is generally known that electrolytic capacitors have a relatively short service life and hence are of low reliability. Specifically, the service life of electrolytic capacitors range from at most 1,000 hours to 2,000 hours.
Though the electrolytic capacitors have such a short life span, they find exceptionally widespread use because of their excellent qualities including the small size, the large capacity, and the low cost, and cannot be replaced with other types of capacitor in many applications.
However, there has been a growing demand for long-life electrolytic capacitors in view of the present trends toward electronic devices of higher performance and higher packing density.
Electrolytic capacitors are usually deteriorated by changes of properties of the electrolytic solution, a degradation of the aluminum oxide film, and other influential elements. Since a loss of electrolytic solution is one of the most responsible for the deterioration of electrolytic capacitors, the service life of electrolytic capacitors can be extended by preventing the electrolytic solution from drying up in use.
It is known that the electrolytic solution is lost through the sealing cap 8 (see FIG. 12A) of rubber. Nevertheless, the sealing cap 8 of rubber is used because it can easily be assembled and also allows a hydrogen gas to permeate itself easily for thereby preventing the aluminum case 9 from being broken due to an internal pressure buildup which is caused by a hydrogen gas generated by the cathode foil 3 when the electrolytic capacitor is aged as described above.
In a general life test, an electrolytic capacitor is placed under environmental conditions including a maximum temperature of usage to measure property deteriorations thereof. Under such static environmental conditions, however, the electrolytic capacitor is tested primarily in a mode in which the electrolytic solution is diffused out through the sealing cap 8 of rubber.
In actual usage, the electrolytic capacitor is energized and heat is generated therein which varies from time to time. Under such dynamic environmental conditions, the electrolytic solution or its vapor tends to leak out beyond the hermetically sealing capability between the lead terminals 1 and the sealing cap 8 of rubber owing to variations in the internal pressure buildup.
It has experimentally confirmed that the service life of an electrolytic capacitor is reduced to one-fifth or one-sixth the expected service life under the above dynamic environmental conditions even if the electrolytic capacitor is used in rated electric ranges.
In some electrolytic capacitors, the limit pressure of the hermetically sealing capability between the lead terminals 1 and the sealing cap 8 of rubber is of about 1.5 atmospheric pressures at an initial stage of usage, but falls to 1 atmospheric pressure or less in about six months due to the stress relaxation of rubber.
When the electrolytic capacitor is mounted on a printed-circuit board, the lead terminals 1 are often subjected to stresses. The lead terminals 1 are greatly strained particularly if there is a large dimensional difference between the distance between the lead terminals 1 and the distance between insertion holes defined in the printed-circuit board which receive the lead terminals 1 therein.
In the presence of such various adverse conditions, the hermetically sealing capability around the lead terminals 1 is impaired to allow the electrolytic solution to leak out, resulting in a reduction in the service life of the electrolytic capacitor.
It has been experienced that the undesirable loss of electrolytic solution tends to invite unexpected accidents of electronic devices which incorporate such electrolytic capacitors.
Recent years have seen wide use of an electrolytic solution containing a solvent of .gamma.-butyrolactone on account of its high performance. The electrolytic solution has a high boiling point of 204 [.degree.C.]. When the electrolytic solution leaks out onto a printed-circuit board, since it does not evaporate well, it brings out an undue conduction between interconnection patterns on the printed-circuit board. Such an undue conduction is liable to cause the circuit to operate out of control and suffer a burnout failure.
There have been proposed various electrolytic capacitors which are designed for protection against electrolytic solution leakage. However, none of the proposed electrolytic capacitors have proven effective enough to prevent the electrolytic solution from leaking out. One of the proposed electrolytic capacitors will be described below with reference to FIG. 13 of the accompanying drawings. Those reference numerals in FIG. 13 which are identical to those shown in FIGS. 11A through lid and 12A through 12C denote identical parts, which will not be described in detail below.
The electrolytic capacitor shown in FIG. 13 has an epoxy resin film 12 disposed on the outer surface of the sealing cap 8 of rubber.
The electrolytic capacitor shown in FIG. 13 is manufactured in the same manner as the process described above with reference to FIGS. 11A through 11D and 12A through 12C, except that, after the electrolytic capacitor has been aged, an epoxy resin in liquid phase is flowed onto the entire exposed surface of the sealing cap 8 of rubber and then hardened into an epoxy resin film 12.
Since the epoxy resin in liquid phase is flowed onto the entire exposed surface of the sealing cap 8 of rubber to form the epoxy resin film 12, the capacitor assembly 7 has to be hermetically encased and sealed in the aluminum case 9 before the epoxy resin in liquid phase is placed on the sealing cap 8. If the capacitor assembly 7 were not hermetically sealed in the aluminum case 9 at the time the epoxy resin in liquid phase is placed on the sealing cap 8, then the epoxy resin in liquid phase would find its way into the capacitor assembly 7.
The epoxy resin used is required to be of cold-setting nature because if it were heated to a high setting temperature, the electrolytic solution would be damaged, and a produced vapor of the electrolytic solution would prevent the epoxy resin from being set.
However, many epoxy resins with good properties are of hot-setting nature. For example, a sealing epoxy resin often used with semiconductor devices can be set at a temperature of 170 [.degree.C.] or higher.
Cold-setting epoxy resins fail to provide a good hermetically sealing capability and hence to completely prevent the electrolytic solution from leaking out. In addition, cold-setting epoxy resins are not sufficiently reliable as to the resistance to high temperatures upon reflow soldering and also as to the intimate contact with the lead terminals.
To solve the above problems which arise out of the use of the epoxy resin, it has been proposed to coat the outer or inner surface of the sealing cap 8 of rubber with a film of fluoroplastics. The film of fluoroplastics is heat-treated when it is coated on the sealing cap 8. Therefore, the electrolytic capacitor is free from the drawbacks which would be caused when the epoxy resin is set, and the electrolytic solution is prevented from leaking through the sealing cap 8.
However, the electrolytic capacitor still suffers other shortcomings. Specifically, when the lead terminals 1 are inserted through holes defined in the sealing cap 8, the capacitor assembly 7 is pressed against the sealing cap 8 under considerable forces. Nonetheless, the lead terminals 1 and the sealing cap 8 are merely held in contact with each other under pressure within the holes in the sealing cap 8. Such a pressure-dependent contact between the lead terminals 1 and the sealing cap 8 is not sufficiently effective to prevent the electrolytic solution from leaking out along the lead terminals 1.